This invention relates in general to the field of strain gages and, more particularly, to a capacitive strain gage and method.
A strain gage is a device used to measure surface strains in structural materials. One type of strain gage used to measure surface strain is a foil type resistance strain gage. Another type of strain gage used to measure surface strain is a capacitive strain gage. A capacitive strain gage generally utilizes capacitors with capacitive plates or elements which are moveable relative to each other as a function of applied strain. As force is applied to the structural material, relative movement of the capacitor elements causes the capacitance to change. The change in capacitance is measured by detecting a change in an applied electrical signal.
Capacitive strain gages, however, generally measure strain in a selected axial or lateral direction. For example, as force is applied to the structural material, movement of the capacitor elements in the selected direction causes a change in capacitance. However, movement of the capacitor elements in a direction other than the selected direction may cause changes in capacitance not associated with applied strain, thereby causing erroneous strain measurements.
Additionally, nonplanar displacement of the structural material may result in erroneous strain measurements or strain gage failure. For example, using a strain gage generally requires securely affixing or bonding the strain gage to the structural material. As forces are applied to the structural material, nonplanar surface displacement of the structural material may cause the strain gage to disbond from the structural material resulting in a loss of strain measurement data.
Usage of strain gages is also generally limited to structural materials having a high modulus of elasticity. For example, foil type resistance strain gages are generally encapsulated in a polyimide resin and attached to a phenolic-type backing material. The backing material is then securely affixed to the structural material using high strength adhesives. The polyimide resin, backing material, and adhesive generally require large forces to elongate the polyimide resin, backing material and adhesive. Thus, conventional strain gages cannot accurately measure strain in low modulus of elasticity materials because the low modulus of elasticity material will elongate prior to elongation of the strain gage.
Further, nonplanar displacement of the structural material may cause erroneous strain measurements. For example, changes in distance between the capacitor elements of a capacitive strain gage in a direction other than the selected measurement direction may result in a change in capacitance unassociated with strain. Therefore, nonplanar displacement of the structural material may result in erroneous strain measurements.
Accordingly, a need has arisen for an improved capacitive strain gage and method that provide greater ease and flexibility of use. The present invention provides a capacitive strain gage and method that addresses shortcomings of prior capacitive strain gages.
According to one embodiment of the present invention, a capacitive strain gage assembly comprises a housing attachable to the component at a first location. A plurality of target members are disposed within the housing. Each target member includes a first target sensor and a second target sensor. A length of the second target sensor is greater than a length of the first target sensor. The strain gage assembly also comprises a sensor member attachable to the component at a second location in moveable relation to the housing. The sensor member is disposed between the target members adjacent the first target sensor of each target member. The sensor member includes a sensor element operable to transmit capacitive signals to the first and second target sensors of each target member.
According to another embodiment of the present invention, a method for fabricating a capacitive strain gage for measuring strain in a component comprises providing a housing attachable to a first location of the component. The method also includes securing a plurality of target members in the housing, each of the target members having a first target sensor and a second target sensor. The length of the second target sensor is greater than the length of the first target sensor of each target member. The method further includes disposing a sensor member attachable to a second location of the component in moveable relation to the housing adjacent the first target sensor of each target member. The sensor member includes a sensor element operable to transmit capacitive signals to the first and second target sensors of each target member.
Embodiments of the present invention provide several technical advantages. For example, one embodiment of the present invention provides a capacitive strain gage assembly that provides greater accuracy than prior capacitive strain gage assemblies by compensating for nonplanar displacement of the structural material. In the same embodiment, the present invention provides increased accuracy of strain measurements for nonplanar structural materials.
Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims.